In communication systems in which a communication medium is shared between multiple communication devices, each device should generally be able to access the shared communication medium in order to send traffic. For example, network devices in Ethernet networks use a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol to access a shared communication medium in the form of a bus. In the event that traffic from more than one device is sent to the bus at substantially the same time, a collision is detected and each device randomly selects an integral delay from a range of delays determined on the basis of a number of attempts that have been made to transmit the traffic. A back-off time interval is then calculated, and the traffic is re-transmitted after the back-off time has elapsed.
In this basic Ethernet protocol, however, there is no priority-based access control for the shared bus. All devices, also commonly referred to as stations or nodes, perform the same algorithm to access the bus and perform a back-off when a collision occurs. This makes it difficult to achieve relative or quantitative Quality-of-Service (QoS) guarantees among different devices or applications being executed on such devices.
Some currently known mechanisms for priority-based access to a shared Ethernet bus alter the delay or back-off interval calculation techniques or back-off algorithms. However, these mechanisms tend to over- or under-prioritize high priority traffic relative to low priority traffic, to such an extent that some of these mechanisms effectively reduce the priority of high priority traffic or explicitly pass or rotate medium access permission.
Other techniques provide for multiple re-transmissions without increasing a range from which back-off times are selected. This type of technique effectively defeats one primary purpose of backing off a communication medium after a failed transmission attempt, in that the probability of collision remains substantially the same for subsequent re-transmissions if a back-off time range is not increased at all.
Priority-based medium access control schemes have been proposed for wireless communication networks, in which the shared communication medium is the air. According to one such scheme, a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism controls access to the wireless medium. If the medium is busy, devices defer their own transmission until the end of a current transmission. After deferral, the devices back off exponentially with a random number of time intervals or slots in the range of 0 and a Contention Window (CW), and decrement a back-off counter only while the medium is idle. Once its back-off time has expired, a device starts transmission. Other devices retain their remaining back-off time until the end of current transmission, and when the medium becomes idle again, continue to decrement their back-off counters. As soon as its back-off counter reaches zero, a device can begin transmission. If a collision occurs, the CW is increased, and a new back-off procedure starts.
QoS support in one proposal for wireless networks is realized by the introduction of an Access Category (AC). Each device may have up to four ACs to support eight user priorities specified by a Virtual Local Area Network (VLAN) Tag. A mapping table maps VLAN priorities to ACs such that an AC with higher priority is assigned a shorter CW and thus in most cases, traffic associated with a higher-priority AC will be transmitted before lower-priority traffic. This is achieved by setting CW limits (CWmin and CWmax) to specified values. For further differentiation, a different inter frame space (IFS), according to AC, is also introduced. A device cannot start a CW when the shared communication medium becomes idle until the IFS expires. Table 1 below shows one example of different values of CW and IFS for each AC:
TABLE 1ACCWminCWmaxIFS0CWminCWmax21CWminCWmax12(Cwmin + 1)/2 − 1CWmin13(CWmin + 1)/4 − 1(Cwmin + 1)/2 − 11
A device selects a random back-off time based on a CW within the CW limits CWmin and CWmax. Back-off time is then calculated as Random(0, CW)*slot time, where slot time depends upon the physical characteristics of the medium. A back-off timer or counter is decremented each time the channel is sensed idle for one time slot. When the back-off timer or counter reaches zero, the device is ready to transmit.
While this approach supports priority-based access to a shared wireless medium, CW limits are not determined based on exponential limit factors. Unlike the CW limits used in this technique, exponential limit factors have relatively well understood characteristics and performance. The introduction of different IFSs further increases the complexity of this scheme.